1. Field of the Invention
This invention relates to the electrochemical reduction of organic disulfides in the presence of halide ion using certain stable solid cathodes to form the corresponding thiols, useful as intermediates in the manufacture of organic pigments and as polymerization activators and polymerization modifiers in the manufacture of rubber. The organic disulfides are formed preferably in situ by starting with an organic sulfonyl halide.
2. Brief Description of the Prior Art
Electrochemical reduction of unsubstituted benzenedisulfides and benzenesulfonyl halides to thiols using a liquid mercury cathode is known in the art, as exemplified in G. M. Nichols, The Electrochemical Society Meeting, New York, 1969, extended Abstracts No. 149, page 364. However, the use of liquid mercury in an industrial process has the disadvantages of being extremely heavy and difficult to handle and tending to create potential water pollution problems.
The use of solid lead and nickel cathodes in the electrochemical reduction of organic sulfonyl halides to thiols is known in the prior art and is exemplified in Fichter and bernoulli, Berichte, Volume 42, page 4308 (1905), and Fichter, Verh. der Naturforsch, Ges. (Basel), Volume 19, page 37 (1908). However, use of these cathodes, although being preferable to the use of a liquid mercury cathode, are disadvantageous because hazardous and toxic titanium chloride salts must be used with the nickel cathode to obtain appreciable yields of thiols; and we have found that the lead cathode, even though producing good yields of thiols, undergoes disintegration during reduction in the presence of halide ion, especially chloride ion. Thus, a continuous or batch type of plant process based on either of these methods is economically unattractive.
It is known in the prior art, as exemplified in Nichols, supra, that electrochemical reduction of organic sulfonyl halides to thiols proceeds through at least three discrete steps involving the intermediates illustrated in the reaction sequence below:
______________________________________ Step Step Step 1 2 3 ______________________________________ RSO.sub.2 X RSO.sub.2 H RSSR RSH sulfonyl sulfinic disulfide thiol halide acid ______________________________________
Where R = aliphatic or aromatic radicals, and X = halogen.
We have found that the reduction of sulfinic acid to disulfide in step 2 occurs at a moderately high electrode potential, whereas the reductions of sulfonyl halide to sulfinic acid in step 1 and of disulfide to thiol in step 3 occur at lower electrode potentials. For the reaction requiring a moderately high electrode potential, a cathode having a moderately high hydrogen overpotential must be used to generate the required electrode potential, whereas, for the reactions requiring lower electrode potentials, cathodes having either moderately high or low hydrogen overpotentials can be used.
Only cathodes having a moderately high hydrogen overpotential and resistance to attack by halide ion can be used for the reduction of sulfonyl halides to thiols. The reason is that in order to effectively reduce sulfonyl halide completely to thiol, the intermediate sulfinic acid formed in step 1 requires a moderately high electrode potential for reduction to the intermediate disulfide in step 2, as illustrated in the above reaction sequence. Furthermore, the cathode must be resistant to attack by halide ion since halide ion is produced in the reduction of sulfonyl halide to sulfinic acid. In the electrochemical production of disulfides to thiols, the cathode also has to be resistant to attack by halide ion, especially chloride ion, because the preferred electrolyte for the catholyte is hydrochloric acid, rather than sulfuric acid or other known electrolytes, since it increases the solubility of the disulfide in the reaction solvent.
Thus a method is needed by the art for the electrochemical reduction of organic disulfides, derived preferably from organic sulfonyl halides, to thiols in the presence of halide ion, using stable solid cathodes, which does not require hazardous reagents, and in which substantial disintegration of the cathodes and heavy metal pollution do not occur.